Growth, thermal and spectral properties of Tm3+, Ho3+ co-doped NaGd(MoO4)2 Crystal

Journal of Alloys and Compounds 615 (2014) 482–487

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Growth, thermal and spectral properties of Tm3+, Ho3+ co-doped NaGd(MoO4)2 Crystal Chunhao Wang a,b, Hao Yin b,c,⇑, Anming Li a,b, Yonghua Wu a,b, Siqi Zhu a,b, Zhenqiang Chen a,b,⇑, Ge Zhang c, Kang Su a,b a b c

Key Laboratory of Optoelectronic Information and Sensing Technologies, Guangdong Higher Education Institutes, Guangzhou 510632, China Institute of Optoelectronic Engineering, Jinan University, Guangzhou 510632, China Key Laboratory of Optoelectronic Materials Chemistry and Physics, CAS, Fuzhou 350002, China

a r t i c l e

i n f o

Article history: Received 18 January 2014 Received in revised form 13 June 2014 Accepted 16 June 2014 Available online 24 June 2014 Keywords: Optical materials Crystal growth Optical properties Tm3+ and Ho3+ dopants NaGd(MoO4)2 crystal

a b s t r a c t A Tm3+/Ho3+:NaGd(MoO4)2 crystal was grown by the Czochralski method. The XRD results matched standard data from JCPDS file, which accorded with scheelite structure with an I41/a space group. Thermal properties of crystal were analyzed by using the TG–DSC curve. The melting point and specific heat are 1182 °C and 0.5 J/g K at 300 K, respectively. The Spectral properties of the Tm3+/Ho3+:NaGd(MoO4)2 crystal were investigated, including room temperature absorption spectrum and fluorescence spectrum. The absorption cross-section of Tm3+ at 796 nm is 4.33  1020 cm2 with pertinent full widths at half maximum (FWHM) of 10 nm. The intensity parameters, spontaneous emission probability, fluorescence branching ratios and radiative lifetimes were calculated by Judd–Ofelt theory. The emission crosssections are 1.47  1020 cm2 and 1.36  1020 cm2 for Tm3+ at 1850 nm and Ho3+ at 2000 nm respectively. The lifetime of 5I7 ? 5I8 (Ho3+) was 4.263 ms. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Owing to the fact that lasing around 2 lm can be intensively absorbed by H2O and CO2, many promising potential applications in medical treatment, military and atmospheric sensing were widely discussed. 3F4 ? 3H6 transition of Tm3+ (1.9 lm) and 5 I7 ? 5I8 transition of Ho3+ (2.1 lm) can be used to realize 2 lm lasing. And Tm3+ can also be doped as sensitizers to transport absorbed pumping energy into Ho3+ efficiently, so Tm3+/Ho3+ codoped system can be pumped by high powered commercial laser diodes. Laser output at 2 lm was successfully obtained in YAG [1], LLF [2], Gd2(MoO4)3 [3], YLF [4], KLu(WO4)2 [5], LiGd(MoO4)2 [6] crystals, tellurite glasses [7] and YAG ceramics [8] with Tm3+/ Ho3+ co-doped system. Tungstate crystals and molybdate crystals with formula MRe(TO4)2 (M, Re stands for alkaline and rare earth metals, respectively; T stand for Mo or W) were reported as promising host materials for solid-state laser [9–12]. This kind of crystals usually has similar scheelite CaWO4 structure. M+ and Re3+ distribute between lattice points of Ca2+ randomly, which can broaden spectral lines ⇑ Corresponding authors at: Institute of Optoelectronic Engineering, Jinan University, Guangzhou 510632, China. Tel.: +86 020 85227066; fax: +86 20 85224387. E-mail addresses: [email protected] (H. Yin), [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.06.084 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

[13]. Most of MRe(TO4)2 crystals can melt congruently. Thus this kind of crystal can be grown by Czochralski method. As a member of MRe(TO4)2 crystals, NaGd(MoO4)2 is regarded as a good solidstate hosting material. The Tm3+ and Ho3+ ion can replace Gd3+ ion easily, so the Tm3+/Ho3+ co-doped system could be introduced in NaGd(MoO4)2 crystal. In this paper, the growth process, thermal and spectral properties of Tm3+/Ho3+:NaGd(MoO4)2 crystal are reported in detail. 2. Crystal growth A Tm3+/Ho3+:NaGd(MoO4)2 single crystal was grown by the Czochralski method. The starting materials Na2CO3(99.5%), Gd2O3(99.99%), MoO3(99.99%), Tm2O3(99.99%) and Ho2O3(99.99%), were weighted according to the following formula:

Na2 CO3 þ 0:02Tm2 O3 þ 0:005Ho2 O3 þ 0:975Gd2 O3 þ 4MoO3 ! 2NaTm0:02 Ho0:005 Gd0:975 ðMoO4 Þ2 þ CO2 "

ð1Þ

Solid state reaction method was used to synthesize polycrystalline Tm3+/Ho3+:NaGd(MoO4)2 material. The weighted raw materials were carefully mixed. During mixing process, extra 2 wt.% MoO3 was added to compensate the volatilization of MoO3, then, well mixed raw materials were extruded into plates, and the plates were put into a crucible. The crucible was placed

C. Wang et al. / Journal of Alloys and Compounds 615 (2014) 482–487

into a box furnace and heated up to 950 °C for 20 h to carry out solid-state reaction. The product was ground into powder. Then the above process was repeated. The synthesized polycrystalline material was placed in a platinum crucible, and was melted in a 2.5 kHz frequency induction furnace. A c-axis NaGd(MoO4)2 crystal bar was used as a seed. After repeated seeding trials, the crystal was grown at a pulling rate of 0.5–1 mm/h, and a rotating rate of 10–30 r/min in an N2 atmosphere. At the ending process, the grown crystal was pulled at rotating rate of 5–10 mm/h; the pulling process was stopped when the crystal was separated from melting surface and then cooled to room temperature, at a rate of 5–30 °C/h. A Tm3+/Ho3+:NaGd(MoO4)2 single crystal with dimension of U 20  25 mm3 was obtained, as shown in Fig. 1. The crystal was brown because of the existence of oxygen vacancies color centers [14–16]. After annealing at 800 °C for 24 h in the air, the crystal became light green. 3. Characterization procedures The concentrations of Tm3+ and Ho3+ ions in Tm3+/Ho3+: NaGd(MoO4)2 were determined by the inductively coupled plasma atomic emission spectrometry (ICP-AES). The X-ray powder diffraction was used to determine the structure and cell constants. A small block of crystal was ground into powder as sample, and the XRD pattern was recorded by a XD-2 diffractometer, using Cu Ka radiation. The acceleration voltage and filament current were 36 kV and 20 mA, respectively. The TG–DSC curve and the specific heat was measured by A NETZSCH STA 449C Simultaneous Thermal Analyzer. The crystal sample was cut into a plate with a dimension of 7  6  1.5 mm and both sides was polished for spectral measurements. The absorption spectrum in the range of 400–2200 nm was measured by a Perkin–Elmer UV–VIS–NIR spectrophotometer (Lambda-900) with the resolution of 1 nm. The fluorescence spectrum and fluorescence lifetime were recorded using a HORIBA Jobin Yvon Triax 550 spectrometer with the resolution of 0.5 nm; a laser diode operating at 780 nm was used as an exciting source for the Tm3+ ions. All of the measurements were conducted at room temperature. 4. Results and discussion 4.1. Concentration and segregation coefficients The concentration of Tm3+ and Ho3+ ion were measured to be 1.301 at.% and 0.345 at.%, respectively. The segregation coefficients (K) of rare earth ions were calculated according to the following equation:

K ¼ C 0 =C 0

483

ð2Þ 0

where the C and C0 are the concentration of rare earth ions in crystal and in the raw materials. The calculated results for Tm3+ and Ho3+ are 0.65 and 0.69. 4.2. X-ray diffraction analysis The XRD pattern of the powder sample was recorded by a diffractometer, which is shown in Fig. 2. The positions and relative intensities of diffraction peaks are in good agreement with CaWO4 (PDF 41-1431) and NaGd(MoO4)2 (PDF 25-0828), which indicate the Tm3+/Ho3+:NaGd(MoO4)2 crystal belongs to classical scheelite (CaWO4) structure with space group I41/a. The doping of Tm3+ and Ho3+ do not change the structure of the host NaGd(MoO4)2 crystal. With the help of cell refinement program of JADE software, the cell constants were estimated to be a = b = 5.219 Å, c = 11.458 Å. Because part of the lattice positions of Gd3+ (ion radii 0.938 Å) are replaced by Tm3+ (ion radii 0.869 Å) and Ho3+ (ion radii 0.894 Å) in host crystal, the cell constant is smaller than pure NaGd(MoO4)2 (a = b = 5.244, c = 11.487, PDF 25-0828). 4.3. Thermal properties The TG–DSC measurement was performed up to 1300 °C at a heating rate of 10 °C/min. Fig. 3 shows the TG–DSC curve of Tm3+/Ho3+:NaGd(MoO4)2 crystal powder sample. Only one endothermic peak (at 1181.5 °C) can be seen in DSC curve, which indicates the crystal melts congruently at 1181.5 °C, and no polymorphic modification or chemical reaction exists in this system. And little mass loss means little volatilization In other words, this crystal has relatively stable chemical property. Generally, a crystal that has higher specific heat means that it has a larger laser damage threshold and lower thermal lens effect. As shown in Fig. 4, the specific heat of the sample at 330 K is 0.51 J/g K. 4.4. Absorption spectrum The absorption spectrum of the polished crystal sample in the range of 400–2400 nm was measured. As shown in Fig. 5, nine relative strong absorption bands can be found, and four of them correspond to transitions of Tm3+ from the 3H6 ground multiplet to 3F2,3 (690 nm), 3H4 (796 nm), 3H5 (1213 nm) and 3F4 (1749 nm) excited multiplets; the other five strong absorption bands correspond to transitions of Ho3+ from the 5I8 ground multiplet to 5G6 (452 nm), 3K8,5F2 (475 nm), 5F4 (541 nm), 5F5 (644 nm) and 5I7 (1952 nm) multiplets. The absorption cross-section, rabs, can be obtained by the formula

Fig. 1. As grown Tm3+/Ho3+:NaGd(MoO4)2 crystal.

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a

41-1431> Scheelite - CaWO4

b 25-0828> Na0.5Gd0.5MoO4 - Sodium Gadolinium Molybdenum Oxide

c 20

30

40

50

60

70

80

2-Theta Fig. 2. The powder XRD pattern of the Tm3+/Ho3+:NaGd(MoO4)2 crystal (a), standard PDF card of CaWO4 (b), and pure NaGd(MoO4)2 crystal (c). Fig. 5. The room temperature absorption spectrum of Tm3+/Ho3+:NaGd(MoO4)2 crystal.

around 2 lm, is 4.33  1020 cm2. Compared with Tm3+:YAG (0.75  1020 cm2) [17], Tm3+:YLF (0.64  1020 cm2) [18] and Ho/ Tm:LuAG (0.57  1020 cm2) [19], the sample has a relatively larger absorption cross-section. The FWHM of absorption band at 796 nm wavelength is about 10 nm. Tm3+/Ho3+:NaGd(MoO4)2 has large absorption cross-section and broad FWHM of absorption band, which indicates the crystal can be pumped efficiently by laser diodes at 780 nm and reduce thermal expansion. 4.5. Judd–Ofelt analysis The Judd–Ofelt theory [20,21] is widely applied in the analysis of spectroscopic properties of rare earth ions in crystals and glasses. From the calculated oscillator strength and radiative transition rates, the J–O intensity parameters Xt (t = 2, 4, 6), radiative lifetimes and fluorescence branching ratios can be determined. According to The J–O theory, the J–O intensity parameters Xt (t = 2, 4, 6) can be determined by a least-square fitting between experimental oscillator strength fexp and calculated oscillator strength fcal. The experimental oscillator strength fexp from the ground multiplet to an excited multiplet can be obtained by

Fig. 3. The TG–DSC curve of Tm3+/Ho3+:NaGd(MoO4)2 crystal.

fexp ¼

mc2

Z

pe2 k2 N0

aðkÞdðkÞ

ð4Þ

Table 1 Absorption cross-sections of Tm3+and Ho3+ions in NaGd(MoO4)2 crystal. Transition Tm

k (nm)

3+ 3

H6?

3

F2,3 H4 H5 3 F4

690 796 1213 1749

1.85 4.33 3.36 2.07

5

452 475 541 644 1952

23.93 6.17 4.10 2.61 2.13

3 3

Ho3+ 5I8?

G6 K8, 5F2 5 F4 5 F5 5 I7 3

Fig. 4. The specific heat of Tm3+/Ho3+:NaGd(MoO4)2 crystal.

rabs ¼

aðkÞ N0

rabs  1020 (cm2)

ð3Þ

where a(k) is the absorption coefficient at wavelength k, and N0 is the density of Tm3+(0.834  1020 cm1) or Ho3+(0.221  1020 cm1). The absorption cross-sections of the nine main absorption bands are listed in Table 1. The absorption cross-section at 796 nm, which is used as pumping channel for laser operating

Table 2 The intensity parameters of Tm3+ and Ho3+ in Tm3+/Ho3+:NaGd(MoO4)2 crystal. Ion name

X2  1020 (cm2)

X4  1020 (cm2)

X6  1020 (cm2)

rms (Df)106

Tm3+ Ho3+

14.61 21.42

2.05 4.30

1.49 1.24

1.25 8.28

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C. Wang et al. / Journal of Alloys and Compounds 615 (2014) 482–487 Table 3 R The mean wavelength, measured and calculated oscillator strength and a(k)dk of Tm3+ and Ho3+ ions in Tm3+/Ho3+:NaGd(MoO4)2 crystal. Transition 3+ 3

Tm

fexp  106 (cm2)

k (nm) 3

H6?

fMD  106 (cm2)

R

a(k)dk (nm/cm)

F2,3 H4 H5 3 F4

690 796 1213 1749

5.13 9.40 5.21 8.99

5.60 9.62 4.07 9.03

– – 0.55 –

18.03 43.99 56.46 203.08

5

452 475 541 644 1952

109.40 11.49 8.14 3.81 2.34

10.26 2.75 4.49 5.02 1.91

– – – – 1.16

43.71 5.07 4.66 3.09 17.46

3 3

Ho3+ 5I8?

fED  106 (cm2)

G6 K8, 5F2 5 F4 5 F5 5 I7 3

Table 4 The spontaneous emission probabilities, fluorescence branching ratios and lifetimes of Tm3+ in Tm3+/Ho3+:NaGd(MoO4)2 crystal. Transition 3+ 3

Wavelength (nm) 3

3

Tm : F4 ? F4? H5?

H6 F4 3 H6 3 H5 3 F4 3 H6 3 F2 3 F3 3 H4 3 H5 3 F4 3 H6

3

1765 4226 1213 2166 1443 796 1634 1494 1167 776 645 473

3

3

H4?

1

G4?

1 AED JJ0 (s ) 3

1.09 * 10 10.89 857.41 76.05 510.32 5.45 * 103 40.84 139.61 1.12 * 103 2.49 * 103 467.91 4.99 * 103

1 ASD JJ 0 (s )

bJJ0

srls

– – 117.14 25.70 – – – – – – – –

1 0.011 0.989 0.017 0.085 0.898 0.005 0.015 0.121 0.269 0.051 0.539

916.810 1.015 * 103 165.890

2.146

Table 5 The spontaneous emission probabilities, fluorescence branching ratios and lifetimes of Ho3+ in Tm3+/Ho3+:NaGd(MoO4)2 crystal. Transition 3+ 5

Ho : I7? I6?

5

5

I5?

5

I4?

5

F5?

5

S2?

5

F4?

5

I8 I7 5 I8 5 I6 5 I7 5 I8 5 I5 5 I6 5 I7 5 I8 5 I4 5 I5 5 I6 5 I7 5 I8 5 F5 5 I4 5 I5 5 I6 5 I7 5 I8 5 S2 5 F5 5 I4 5 I5 5 I6 5 I7 5 I8 5

Wavelength (nm)

1 AED JJ 0 (s )

1 ASD JJ0 (s )

bJJ0

sr (ls)

1952 2858 1173 3925 1654 903 5053 2199 1235 753 4214 2298 1445 967 651 3650 1956 1410 1011 754 550 – 3305 1852 1355 1005 754 550

155.57 45.82 333.09 20.72 146.18 113.26 11.63 60.08 71.08 14.55 0.23 20.31 242.18 1.38 * 103 4.83 * 103 1.51 32.06 57.81 316.73 1.41 * 103 1.80 * 103 – 83.55 43.18 305.40 852.23 1.36 * 103 7.4 * 103

93.49 26.95 – 12.18 – – 4.73 – – – – – – – – – – – – – – – 8.84 – – – – –

1 0.179 0.821 0.113 0.500 0.387 0.101 0.371 0.439 0.09 0 0.003 0.037 0.213 0.746 0 0.009 0.016 0.088 0.389 0.498 0.009 0.004 0.030 0.085 0.132 0.739

4.015 * 103 2.464 * 103

where m and e are the mass and charge of electron, respectively; c is velocity of light, k is the mean wavelength of the absorption band, R N0 is the number density of rare earth ion, a(k)dk is the integral of the absorption coefficient in the corresponding absorption band. The calculated oscillator strength fcal is the sum of the electricdipole (ED) oscillator strength and magnetic-dipole (MD) oscillator strength.

4.043 * 103

6.17 * 103

154.559

276.387

99.617

fcal ¼ f ED þ f MD

ð5Þ

According to the J–O theory the electric-dipole (ED) oscillator strength fED should be

f ED ¼

2 8p2 mc ðn2 þ 2Þ X Xt jh4f n SJLjjU t jj4f n S0 J0 L0 ij2 3hð2J þ 1Þk 9n t¼2;4;6

ð6Þ

486

C. Wang et al. / Journal of Alloys and Compounds 615 (2014) 482–487

Fig. 6. The room temperature fluorescence spectrum of Tm3+/Ho3+:NaGd(MoO4)2 crystal.

Fig. 8. The decay curve of Tm3+/Ho3+:NaGd(MoO4)2 crystal for the 5I7 ? 5I8 (Ho3+).

f MD ¼

2p2 n jh4f n SJLjjL þ 2Sjj4f n S0 J 0 L0 ij2 3hmcð2J þ 1Þ

ð8Þ

where the jh4f n SJLjjL þ 2Sjj4f n S0 J0 L0 ij2 is the reduced matrix element for the operator L + 2S and the calculation procedure has been reported in reference [24]. For the absorption bands which include two transitions, 3H6 ? 3F2,3 and 5I8 ? 3K8,5F2, the sums of the corresponding matrix elements were used. In order to evaluate the accuracy of least square fitting, the root mean square (rms) was calculated using follow formula

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX 2 rmsðDf Þ ¼ t ½fexp  ðf ED þ f MD Þ =ðN  3Þ

ð9Þ

i¼1

Fig. 7. Schema of energy transfer of Tm3+/Ho3+:NaGd(MoO4)2 crystal.

where h is the Plank constant, n is the refractive index; J is the total angular momentum of the initial state, jh4f n SJLjjU t jj4f n S0 J0 L0 ij are the reduced matrix elements. In this work the reduced matrix elements of Tm3+ and Ho3+ were taken from Ref. [22]. The refractive index can be calculated by Sellmeier equation

n¼Aþ

Bk

AJJ0 ¼ AED þ AMD

AED JJ 0 ¼

2

k2  C

where N is the number of absorption bands used in the above calR culations. The values of Xt, rms (Df), fexp, fED, fMD and a(k)dk are listed in Tables 2 and 3. The intensity parameters of Tm3+ and Ho3+ in Tm3+/Ho3+:NaGd(MoO4)2 are relative closed to other molybdate crystals [6,12]. From the J–O intensity parameters, the spontaneous emission probability AJJ0 , fluorescence branching ratios b and radiative lifetimes sr of excited multiplets were calculated. Taking into account the magnetic-dipole transitions, the spontaneous emission probability A should be

 Dk2 2

ð7Þ

where A, B, C and D are Sellmeier parameters, k is the wavelength. The Sellmeier parameters of NaGd(MoO4)2 crystal were taken from Ref. [23]. When the transition meets the selection rule of DS = DL = 0, DJ = 0, ±1, the magnetic dipole transitions between the states of 4fnconfiguration are allowed. Then magnetic-dipole (MD) oscillator strength can be calculated by formula:

AMD JJ 0 ¼

ð10Þ

2 nðn2 þ 2Þ X Xt jh4f n SJLjjU t jj4f n S0 J0 L0 ij2 9 3hð2J þ 1Þk3 t¼2;4;6

64p4 e2

64p4 e2 3hð2J þ 1Þk3

n3 jh4f n SJLjjL þ 2Sjj4f n S0 J 0 L0 ij2

ð11Þ

ð12Þ

MD where AED are the electric-dipole spontaneous emission JJ 0 and AJJ 0 probability and the magnetic-dipole spontaneous emission probability, corresponding to transitions from a J multiplet to a J0 multiplet, respectively. The reduced matrix elements were taken from

Table 6 Spectral properties of Tm3+/Ho3+ co-doped YAG, YLF, LuAG, NaY(WO4)2 and NaGd(MoO4)2 crystal. Crystal

kabs (nm)

rabs  1020 (cm2)

FWHM of absorption band (nm)

kem (lm)

rem  1020 (cm2)

FWHM of emission band (nm)

Refs.

YAG YLF LuAG NaY(WO4)2 NaGd(MoO4)2

780 780 788 795 796

0.75 0.74 0.57 4.10 4.33

4 8 10 9 10

2.097 2.045 2.100 2042 2.080

0.7 0.87 1.13 1.43 1.36

30 29.4 50 – 50

[17] [27] [19,28] [29]

C. Wang et al. / Journal of Alloys and Compounds 615 (2014) 482–487

Refs. [25,26]. The fluorescence branching ratios bJJ0 and radiative lifetimes sr can be calculated by the following formula:

AJJ0 bJJ0 ¼ P 0 J 0 AJJ

sr ¼ P

ð13Þ

1 J 0 AJJ

ð14Þ

0

The values of these spectroscopic parameters for Tm3+ and Ho3+ are all listed in Tables 4 and 5, respectively. 4.6. Fluorescence spectrum The room temperature fluorescence spectrum of Tm3+/Ho3+: NaGd(MoO4)2 crystal around 2.0 lm, excited with 780 nm radiation, is shown in Fig. 6. The emission band consists of 3F4 ? 3H6 transition of Tm3+ions and 5I7 ? 5I8 transition of Ho3+ ions, and the width of the fluorescence band can reach about 350 nm. A broad emission band indicates that the Tm3+/Ho3+:NaGd(MoO4)2 crystal can be used as a tunable laser crystal. The emission of Tm3+ ion at about 1850 nm is stronger than emission of Ho3+ at about 2000 nm, which indicates the energy transfer from Tm3+ to Ho3+ is not complete. The energy transfer process can be expressed in Fig. 7. Under the exciting of 780 nm laser source, Tm3+ ion is excited into the upper multiplet 3H4, then Tm3+ reaches 3F4 by the procedure of cross relaxation (3H4 + 3H6 ? 3F4 + 3F4). Part of energy in multiplet of 3F4 are transferred into 5I7 multiplet of Ho3+, and Ho3+ decays radiatively from 5I7 to 5I8, generating the emission around 2000 nm; the other part of the energy transfer to 3H6 directly, generating the emission around 1085 nm. The decay curve of Tm3+/Ho3+:NaGd(MoO4)2 crystal for the 5 I7 ? 5I8 (Ho3+) was measured, as shown in Fig. 8. The lifetime was obtained as 4.263 ms by fitting the single exponential decay curve. The emission cross-section rem of Tm3+ and Ho3+ were determined by the McCumber theory

rem ¼ rabs exp

Ezl  hck1 kB T

! ð15Þ

where Ezl is the zero line energy; kB is Boltzmann constant; T is the temperature (in this work, 300 K was taken as room temperature). The Ezl of Tm3+ and Ho3+ are 5646 cm1 and 5029 cm1, both of them were taken from Ref. [22]. The rem of Tm3+ is1.47  1020 cm2 at around 1850 nm, the rem of Ho3+ is1.36  1020 cm2 at around 2000 nm. In order to make further evaluation on spectral properties of Tm3+/Ho3+:NaGd(MoO4)2 crystal, the spectral properties of Tm3+/ Ho3+ co-doped YAG, YLF, LuAG, NaY(WO4)2 and NaGd(MoO4)2 crystal were compared and listed in Table 6. The absorption cross-section and emission cross-section of Tm3+/Ho3+ co-doped NaGd(MoO4)2 are similar to double tungstate NaY(WO4)2 and higher than that of YAG, YLF, LuAG. Compared with other crystals in Table 6, the absorption band and emission band of the Tm3+/ Ho3+:NaGd(MoO4)2 crystal are broad enough. 5. Conclusions A Tm3+/Ho3+ co-doped NaGd(MoO4)2 crystal was grown by Czochralski method and its tetragonal structure was identified. Detailed spectrum properties of the crystal were investigated,

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including room temperature absorption spectrum and fluorescence spectrum. The absorption cross-section of Tm3+ at 796 nm is 4.328  1020 cm2 with FWHM of 10 nm. The Judd–Ofelt analysis of Tm3+ and Ho3+ was presented. The broad emission band around 2000 nm, which is about 350 nm width, indicates the Tm3+/Ho3+: NaGd(MoO4)2 crystal can be a promising candidate as a tunable laser crystal. The emission cross-section of Tm3+ at 1850 nm is 1.47  1020 cm2 and the emission cross-section of Ho3+ at 2000 nm is 1.36  1020 cm2. The lifetime of 5I7 ? 5I8 (Ho3+) was obtained as 4.263 ms. The results indicate the Tm3+/Ho3+: NaGd(MoO4)2 crystal is a potential solid-state laser material. Since the emission band of Tm3+ is stronger than that of Ho3+, further work should be done to improve energy transfer efficiency. Acknowledgements Chunhao Wang and Hao Yin contribute equally to this paper. The work is supported by Natural Science Foundation of Guangdong Province (S2013040016819), the project of IndustryAcademia-Research of Guangdong Province (2010B090500022), and Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016). References [1] T.Y. Fan, G. Huber, R.L. Byer, P. Mitzscherlich, Opt. Lett. 12 (1987) 678–680. [2] X.L. Zhang, S. Zhang, N.N. Xiao, J.H. Cui, J.Q. Zhao, L. Li, Appl. Opt. 53 (2014) 1488–1492. [3] L.L. Yang, J.F. Tang, J.H. Huang, X.H. Gong, Y.J. Chen, Y.F. Lin, Z.D. Luo, Y.D. Huang, Opt. Mater. 35 (2013) 2188–2193. [4] R.K. Feaver, R.D. Peterson, P.E. Powers, Opt. Express 21 (2013) 16104–16110. [5] V. Jambunathan, X. Mateos, M.C. Pujol, J.J. Carvajal, U. Griebner, V. Petrov, M. Aguiló, F. Díaz, Opt. Laser Technol. 54 (2013) 326–328. [6] J.F. Tang, Y.J. Chen, Y.F. Lin, X.H. Gong, J.H. Huang, Z.D. Luo, Y.D. Huang, Opt. Mater. Exp. 2 (2012) 1064–1075. [7] G.X. Chen, Q.Y. Zhang, G.F. Yang, Z.H. Jiang, J. Fluoresc. 17 (2007) 30–3071. [8] W.B. Liu, Y.P. Zeng, L. Wang, Y.Q. Shen, B.X. Jiang, J. Li, D. Zhang, Y.B. Pan, Laser Phys. 22 (2012) 1622–1626. [9] W. Zhao, W.W. Zhou, R.Z. Zhuang, G.F. Wang, J.M. Du, H.J. Yu, Z.C. Lv, F.W. Wang, Y.H. Chen, Mater. Res. Innov. 16 (2012) 237–242. [10] X.Y. Huang, W. Zhao, G.F. Wang, X.X. Li, Q.M. Yu, J. Alloys Comp. 509 (2011) 6578–6584. [11] Y.W. Wei, Y.J. Chen, Y.F. Lin, X.H. Gong, Z.D. Luo, Y.D. Huang, J. Alloys Comp. 484 (2009) 529–534. [12] W.W. Zhou, B. Wei, W. Zhao, G.F. Wang, X. Bao, Y.H. Chen, F.W. Wang, J.M. Du, H.J. Yu, Opt. Mater. 34 (2011) 56–60. [13] Z.J. Wang, X.Z. Li, Q. Wei, X.F. Long, Mater. Res. Innov. 12 (2008) 174–178. [14] Z.J. Wang, X.Z. Li, G.J. Wang, M.J. Song, Q. Wei, G.F. Wang, X.F. Long, J. Lumin. 128 (2008) 451–456. [15] X.Y. Huang, Z.B. Lin, L.Z. Zhang, G.F. Wang, J. Cryst. Growth 306 (2007) 208– 211. [16] G.M. Kuz’Micheva, D.A. Lis, K.A. Subbotin, V.B. Rybakov, E.V. Zharikov, J. Cryst. Growth 275 (2005) e1835–e1842. [17] T.Y. Fan, G. Huber, R.L. Byer, P. Mitzscherlich, IEEE J. Quantum Elect. 24 (1988) 924–933. [18] I. Razumova, A. Tkachuk, A. Nikitichev, D. Mironov, J. Alloys Comp. 225 (1995) 129–132. [19] N.P. Barnes, E.D. Filer, F.L. Naranjo, W.J. Rodriguez, M.R. Kokta, Opt. Lett. 18 (1993) 708–710. [20] B.R. Judd, Phys. Rev. 127 (1962) 750–761. [21] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. [22] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424–4442. [23] X. Han, D.E. Lahera, M.D. Serrano, C. Cascales, C. Zaldo, Appl. Phys. B 108 (2012) 509–514. [24] B.M. Walsh, Springer, Netherlands, 2006. [25] N. Spector, R. Reisfeld, L. Boehm, Chem. Phys. Lett. 49 (1977) 49–53. [26] E. Rukmini, C.K. Jayasankar, Opt. Mater. 4 (1995) 529–546. [27] C. Li, Y. Zhang, X.J. Zhang, F.M. Zeng, M. Tonelli, J.H. Liu, J. Rare Earths 29 (2011) 592–595. [28] K. Scholle, E. Heumann, G. Huber, Laser Phys. Lett. 1 (2004) 285–290. [29] C.L. Sun, F.G. Yang, T. Cao, Z.Y. You, Y. Wang, J.F. Li, Z.J. Zhu, C.Y. Tu, J. Alloys Comp. 509 (2011) 6987–6993.